Various processes for producing an aromatic polyimide separating membrane are known. For example, Japanese patent application Laid-open No. 49-45152 discloses a process for producing an aromatic polyimide asymmetrical membrane, in which process a dope solution consisting of a homogeneous solution of an aromatic polyamic acid is spread to form a thin layer thereof and the thin dope solution layer is coagulated in a coagulating liquid containing a lower aliphatic carboxylic acid or its anhydride and a tertiary amine while imide-cyclizing the amide-acid structure in the polyamic acid into the corresponding imide structure.
In the above-mentioned known process, however, since the coagulating procedure of the dope solution layer and the imide-cyclizing procedure of the amide-acid structure are simultaneously carried out in the coagulating liquid, it is difficult to obtain an aromatic polyimide separating asymmetrical membrane having an excellent, stable gas-separating property with a satisfactory degree of reproductivity. That is, the gas-separating property of the resultant polyimide asymmetrical membrane is unstable and varies widely. In addition, the resultant polyimide asymmetrical membrane exhibits an unsatisfactory gas-permeating rate.
Japanese patent application Laid-open No. 50-99971 also discloses a process for producing an aromatic polyimide semi-permeable membrane by means of a dry membrane-forming method. Furthermore, Japanese patent application Laid-open No. 56-21602 discloses a process for producing a biphenyl tetracarboxylic acid-type aromatic polyimide semi-permeable membrane by means of a wet membrane-forming method.
The aromatic polyimide membranes produced by means of the known processes, however, exhibit an unsatisfactory gas-separating property. Also, in the case of the aromatic polyimide separating membrane produced by means of the known processes, when the membrane exhibits a satisfactory gas-separating property, usually, the gas permeability of the membrane is unsatisfactory, and when the membrane exhibits a satisfactory gas permeability, usually the gas-separating property of the membrane is unsatisfactory. That is, in the prior arts, it is difficult to obtain an aromatic polyimide membrane having both a satisfactory gas permeability and a satisfactory gas-separating property.
Usually, the intensity of the gas-separating property of the aromatic polyimide membrane is represented by the ratio (.rho.H.sub.2 /.rho.CO) of the hydrogen gas permeability (.rho.H.sub.2) to the carbon monoxide gas permeability (.rho.CO) of the membrane.